I checked the definitive books that Fr. Zephyrin Engelhardt wrote on the San Gabriel Mission (published in 1927) and the San Fernando Mission (1922) but they do not provide any details. The San Gabriel Mission book does have a map of the "Old California Missions" but it is very general.
There are several possible sources I can suggest depending on how much time you want to devote to this. There are early maps of California at the Library of Congress and the University of California at Berkeley. You might also find early maps of both towns that could help. The Mission Archive Library at Santa Barbara is another possible source. (http://www.sbmal.org/ )The California Mission Studies Association sells a large map showing the mission locations but there may not be enough detail on the exact route between san Gabriel and San Fernando (http://www.ca-missions.org/pubs.html#map ).
There was a lot of research done on the evolving road between the mission (El Camino Real) at the beginning of the 20th century when hundreds of bell markers were placed between 1906-1915 (for more details on this project see http://www.cahighways.org/elcamino.html ).